A. Technical Field
The present invention relates generally to real-time clock circuits, and more particularly to methods, systems and devices that employ two oscillators in a real-time clock circuit to generate accurate time of day over an industrial temperature range for electronic applications.
B. Background of the Invention
Time of day is tracked by a real-time clock (RTC) circuit in computers and embedded systems. In addition to a primary power source, the RTC circuit requires a secondary power source which is normally a lithium battery to track the time of day continuously when the primary power source is discontinued. The core of the RTC circuit is a crystal oscillator that has a typical resonant frequency of 32.768 kHz. Such a crystal oscillator is also used in quartz clocks and watches, and therefore the particular crystal in the oscillator is also called a “watch crystal.” Since this watch crystal generates 215 clock cycles per second, a RTC circuit based on the watch crystal may be easily implemented using binary counter circuits for use in various electronic applications. Moreover, the watch crystal needs low power consumption that can be easily sustained by both of the primary and secondary power source.
Some electronic applications may impose stringent requirements on the accuracy of the time measurement that is provided by the RTC circuit. For instance, in an electric powermeter, accuracy specifications for the time of day are such that time drift within one day has to be less than 5.78 ppm (i.e., 0.5 second/day) at the room temperature (25° C.) and less than 11.57 ppm (i.e. 1 second/day) over an industrial temperature range of [−25° C., 60° C.]. These accuracy specifications are also adopted in various electronic devices, and some devices even requires the latter specification on drift of time of day within one day, 11.57 ppm, to be applied to a wider temperature range, [−40° C., 85° C.].
In order to maintain a highly accurate time of day, the oscillator circuit needs to compensate the temperature drift caused by the watch crystal. The watch crystal is normally built in a tuning fork configuration. The resonant frequency (32.768 kHz) of the watch crystal reaches a peak at a turnover temperature Tt, and drops as the temperatures increases or decreases, resulting in a significant quadratic error. This error ERR(T) may be represented asERR(T)=A+Q(T−Tt)2 ppm  (1)where A is initial error tolerance in ppm, and Q is quadratic coefficient in ppm/° C.2. Typical manufacturing limits are 25° C.±5° C. for Tt and −0.036 ppm/° C.2±10% for Q, respectively. Since a frequency is the reciprocal of the period of a clock cycle, a drift of the resonant frequency is equal to the drift of time of day, and the frequency drift may be thus used to represent the drift of the time of day associated with the particular RTC circuit.
FIG. 1A illustrates the error of the resonant frequency 100 in various watch crystals. Curve 102 is associated with a nominal watch crystal, while curves 104-106 and curves 108-110 are associated with two corner cases of the turn over temperature, Tt. For each corner case of Tt, two corner cases of the quadratic coefficient, Q, are presented as well. In particular, in watch crystals having a large quadratic coefficient Q, the error reaches up to −120 ppm at −25° C. If an error tolerance of [−10 ppm, 10 ppm] is imposed on the resonant frequency, most watch crystals may only work within a temperature range that is much narrower than either of the above industrial temperature ranges.
A convenient solution is to integrate a temperature measurement circuit in the oscillator circuit, and compensate the resonant frequency to the nominal value at the room temperature (close to Tt). FIG. 1B illustrates the temperature-compensated error of the resonant frequency 150 in various watch crystals in reference to the nominal watch crystal. The nominal watch crystal has a flat error at 0 ppm due to temperature compensation, and thus, the initial error at 25° C. is trimmed to zero. Curves 154-156 and curves 158-160 are associated with two corner cases of turn over temperature, Tt. Therefore, the temperature-compensated error of the resonant frequency falls below 30 ppm at −25° C. which is still beyond the aforementioned requirement of 11 ppm. Thus, in order to meet an 11 ppm spec, watch crystals must be individually characterized so that the temperature compensation circuit may be properly programmed.